Field of the Invention
The present disclosure relates to a light-emitting element, a light-emitting element array, an exposure head, and an image formation apparatus.
Description of the Related Art
An example of electrophotographic printers uses an exposure head to expose a photoconductor drum and form a latent image. The exposure head includes a light-emitting element array in which semiconductor light-emitting elements, such as light-emitting diodes (LED), are arranged in a longitudinal direction of a photoconductor drum, and a rod lens array which focuses light emitted from the light-emitting element array on the photoconductor drum. A printer employing an exposure head is attracting attention because of the advantages that reduction in size is easy, for example, as compared with a printer employing a laser scanning system in which deflection scanning is conducted using a laser beam with a polygon mirror.
An example of the light-emitting element array is a self-scanning light-emitting thyristor array. The self-scanning light-emitting thyristor array includes a shift thyristor in which thyristors are arranged unidimensionally as switch elements, and a light-emitting thyristor in which thyristors are arranged unidimensionally as light-emitting elements, which are integrated on the same substrate.
Japanese Patent Laid-Open No. 2013-58789 describes providing a current confinement mechanism in each light-emitting thyristor by oxidizing a part of a semiconductor layer in a self-scanning light-emitting thyristor array. This configuration concentrates a current on a part of the light-emitting thyristor, and improves light output power as compared with a case where no current confinement mechanism is provided.
The light-emitting element array of Japanese Patent Laid-Open No. 2013-58789 will be described with reference to FIGS. 11A to 11C. FIG. 11A is a simplified plan view of a part of the light-emitting element array of Japanese Patent Laid-Open No. 2013-58789. The light-emitting element array of Japanese Patent Laid-Open No. 2013-58789 includes a light-emitting thyristor L as a light-emitting element, a shift thyristor T, a parasitic thyristor P, and a common gate G of each thyristor. These are formed on a single mesa 1010 which has side surfaces 1 to 4.
FIG. 11B is a cross-sectional view of the shift thyristor T along line XIB-XIB of FIG. 11A. The shift thyristor T has a pn-pn thyristor structure in which a p-AlGaAs layer 1001, a p-AlGaAs layer 1002 (Al composition: about 0.98), a p-AlGaAs layer 1003, an n-AlGaAs layer 1004, a p-AlGaAs layer 1005, and an n-AlGaAs layer 1006 are stacked in this order on a p-GaAs substrate 1000. The p-AlGaAs layers 1001, 1002, and 1003 are considered to be anodes, the n-AlGaAs layer 1004 is considered to be an n gate, the p-AlGaAs layer 1005 is considered to be a p gate, and the n-AlGaAs layer 1006 is considered to be a cathode. The shift thyristor T includes a cathode electrode 1007 disposed on a front surface (the n-AlGaAs layer 1006), and an anode electrode 1009 disposed on a back surface.
The p-AlGaAs layer 1002 is partially oxidized and has high resistance at the oxidized portion. That is, the p-AlGaAs layer 1002 has oxidized regions 1002A and a non-oxidized region 1002B. The mesa 1010 is formed so that the p-AlGaAs layer 1002 is exposed, and the mesa 1010 is oxidized from side surfaces 1 to 4 of the mesa 1010. Then, the oxidized regions 1002A are formed. Then, the non-oxidized region 1002B which is substantially the same in shape with the mesa 1010 is formed inside of the mesa 1010. Although it depends on the crystal orientation of the mesa 1010, a depth to which oxidization is conducted from the side surfaces 1 to 4 is equally the width d if the four side surfaces 1 to 4 of the mesa 1010 have equivalent crystal orientation. In this manner, the non-oxidized region 1002B is formed so as to be surrounded by the oxidized region 1002A of the width d from the side surfaces 1 to 4 of the mesa 1010. Since resistance of the oxidized region 1002A is higher than resistance of the non-oxidized region 1002B, if a current is made to flow in a stacked direction of semiconductor, the current substantially flows through the non-oxidized region 1002B with concentration. With this configuration, a region which can emit light is limited to the non-oxidized region 1002B.
FIG. 11C is a cross-sectional view along line XIC-XIC of FIG. 11A including the light-emitting thyristor L and the shift thyristor T. As described above, the shift thyristor T and the light-emitting thyristor L are the same in semiconductor layer configuration. When both of these thyristors are turned on, a current flows between the anode and the cathode (in a stacked direction of the semiconductor), and a region of the multilayer structure in which the current flows emits light.
A light-emitting element array as a light source of an exposure head needs to have high contrast as the entire light-emitting element array in order to form a sharp and high definition image. That is, it is needed that when the light-emitting element is turned on, there is no light emission from elements other than the light-emitting element, or it is needed that light emission from the light-emitting element is sufficiently larger than light emission from elements other than the light-emitting element.